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Even before it kicks off in earnest, the assault on Somalia's port city of Kismayo is causing jitters.

A preliminary push by UN-backed African Union troops on the last bastion of Somalia's Al-Shabab insurgency has already added to Somalia's civilian casualties, and there are fears that more may lie ahead as air, ground and naval operations in the strategic city escalate.

The latest, and most senior, person to raise the alarm over the actions of the Kenyan Defence Forces (KDF), which officially operate under the banner of the African Union Mission in Somalia (AMISOM), was UN Resident and Humanitarian Coordinator Mark Bowden.

"I am deeply concerned by recent reports of civilian casualties in Kismayo caused by naval gunfire and airstrikes," Bowden said in a statement issued on 14 August.

"As fighting for control of the town appears imminent, I reiterate my call for all parties to the conflict to make every effort to minimize the impact of conflict on civilians and to allow full humanitarian access to all people in need," he said.

An NGO official working on Somalia told IRIN that aid workers were worried there would be "an increase in indiscriminate shelling and large-scale civilian casualties.

"Furthermore, civilians may get trapped in between fighting forces, further restricting their access to life-saving support and humanitarian assistance in an area that is of great humanitarian need - need that is barely properly assessed due access restrictions," said the official.

Al-Shabab has banned many aid agencies from Kismayo, and general insecurity in the area has also impeded humanitarian access.

The city has a population of 90,000, with another 77,000 living in surrounding areas. Thanks to illegal exports of charcoal, the port is a major source of the insurgency's income. The city is also likely to become the capital of a proposed semi-autonomous region under the country's imminent federal government - a region sometimes referred to as the Jubaland Initiative.

International humanitarian law

Human rights organizations say all parties to Somalia's conflict - AMISOM, the forces of the outgoing Transitional Federal Government (TFG), Al-Shabab and other armed groups - have violated International Humanitarian Law, chiefly through the use of indiscriminate fire. Between August 2010 and August 2011, there were 4,000 combat-related civilian casualties in Somalia, including 1,000 deaths, according to Human Rights Watch (HRW).

(AMISOM is now in the process of setting up a Civilian Casualty Tracking, Analysis and Response Cell).

In October 2011, the medical relief agency Médécins Sans Frontières (MSF) said its staff treated dozens of injured following an aerial bombardment on the town of Jilib that hit a camp for internally displaced people.

Kenya, which had deployed militarily in Somalia two weeks earlier, said it had only targeted Al-Shabab positions.

Since then, KDF forces have been "re-hatted" under AMISOM.

"AMISOM is cognizant of its responsibilities under International Humanitarian Laws," the mission's senior spokesman, Eloi Yao told IRIN.

"As such, it has adopted a policy on the use of indirect fire weapons and has trained its peacekeepers on the same. As in operations in Mogadishu and its environs, all precautions are being observed, including the establishment of no-fire zones for use of artillery in populated areas and around schools, mosques and markets," he said.

But, he added, when asked about KDF's weekend bombardment of Kismayo, "Not all air assets operating in the Somali airspace are under [AMISOM's ] command."

One factor that could limit civilian casualties is Al-Shabab's recent history of avoiding major engagement with advancing AMISOM forces. As they did in Mogadishu in mid-2011, the insurgents are expected by some analysts to withdraw from Kismayo rather than put up a fight.

When AMISOM/TFG took control of the town of Afgoye, near Mogadishu, in May 2012, they took steps to reduce civilian casualties by choosing the routes of attack into the corridor carefully and by requiring multiple verifications of targets.

Yet, as one humanitarian source explained, that operation also disrupted humanitarian activities and displaced tens of thousands of people, many of whom fled in anticipation of the actual fighting.

Urgent needs

While there are no recent detailed assessments of the humanitarian needs in Kismayo, it is clear that the dearth of aid agencies and the presence of Al-Shabab have caused significant hardship.

Residents reached by phone told IRIN that a lack of affordable food, clean water and healthcare, as well as restrictions on their movements, were among the main problems they faced.

One inhabitant said, "Families have fled from Kismayo, and they are still fleeing. My family is among those who are fleeing."

HRW has received reports that Al-Shabab was attempting to block the movement of people out of the city.

According to the UN Refugee Agency, over 800 people have been displaced from Kismayo over the last three weeks. Unconfirmed reports point to increased displacement from the town in the last 24 hours. Before this upsurge, many people were leaving more because of a lack of humanitarian assistance than fear of military intervention, the agency's population movement tracking data indicates.

A full assessment of the population will only be possible when the situation on the ground permits. UN Department of Safety and Security clearance is required before UN agencies can operate in any area.

According to UNOSAT, in mid-May there were about 4,000 dwellings for internally displaced people in two dozen sites around the city.

MSF, which conducts emergency operations in Kismayo, said in July that cases of acute watery diarrhoea were on the rise, in part due to a ban on the use of chlorine.

"Kismayo's dense population means that a widespread outbreak could easily occur if urgent prevention measures are not taken," MSF said in a statement, noting that its own limited resources had been "overloaded" by patients and that "a larger facility was needed to treat the increasing number of cases."

The Al-Shabab-controlled general hospital was recently reported to be admitting some 40 suspected cases of cholera every week.

Omar Saleh, the World Health Organization's emergency health coordinator for Somalia told IRIN other concerns in Kismayo included a lack of trained health personnel and that, because of access problems, "no vaccinations had been carried out since 2009".

He said there was a "very high risk" of mass casualties due to communicable diseases such as cholera and measles.

Saleh said the top health priorities now were increasing "access to safe water and sanitation, and to medical supplies, emergency surgery and trauma kits. A humanitarian corridor has to be negotiated and created."

Kismayo after Al-Shabab

Whether Al-Shabab is routed from Kismayo or withdraws quietly, few believe its departure will lead to immediate stability.

"International military interventions in Somalia have been plagued by unforeseen consequences," said Chatham House analyst Adjoa Anyimadu.

"No one's giving much attention to the idea that civilians suffering under Al-Shabab could find they are suffering again" once they leave, he said.

Laetitia Bader, a researcher at HRW, says there has been a rise in insecurity in several southern Somali towns, including Beledweyne, Baidoa and Afgoye, since Al-Shabab left them.

"Donors see the withdrawal of the Al-Shabab as an opportunity, but they need to be aware of the other actors they'll be dealing with," she said.

Documented crimes by other actors, including pro-government militia groups, include summary execution.

Other NGOs have noted a decline in security in "liberated" towns and warn that for humanitarians, even if towns are under government control, many of the rural areas surrounding them - such as the hills overlooking Kismayo - could remain under Al-Shabab control, hampering any immediate improvement in humanitarian access.

Kismayo was fought over by rival clans long before Al-Shabab emerged as a player. "Its strategic importance, as an all-weather port in Somalia's most fertile region, ensures a strong position for those governing the city," said James Smith, an analyst with Rift Valley Institute.

"Al-Shabab's presence has guaranteed a degree of stability in Kismayo. If forced out, competition for control of the city may result in inter-clan conflict in the region," said Smith.

One aid worker who asked not to be named said there was a danger of a "governance vacuum in Kismayo, as various forces will vie for control, influence and power. It would be good to know what governance plans are being drawn up."

At the national level, the TFG's mandate expires on 20 August, when new, theoretically more sustainable, governance structures take hold.